Case-hardening (carburization) is often used to produce components which are subjected to fatigue stresses. This carburization gives the component a hard, carbon-rich surface at the same time as a tough, low-carbon core is obtained after hardening and annealing. Such a component has a very good resistance to wear and fatigue. The profile of hardness/residual stress obtained is schematically illustrated in the accompanying FIG. 1, and the surface residual stresses obtained are very favorable from a fatigue life point of view.
However, case-hardening is a rather expensive and slow process. Therefore, many attempts have been made to make components with acceptable wear and fatigue resistance by surface hardening using induction heating. In such attempts, a low-alloyed steel having a higher content of carbon, e.g., 0.5%-0.6%, is normally used and the surface zone of the component is heated rapidly, whereupon the component is subjected to quenching. The profile of hardness/residual stress then obtained is illustrated schematically in the accompanying FIG. 2.
This profile of hardness/residual stress obtained from a surface induction hardened component in combination with the low alloy steels used is sufficient in several situations, but often gives a limited contact fatigue strength. It also gives rise to problems under other fatigue conditions. One factor contributing to the often inferior properties is the reduction of hardness which is experienced at a distance below the surface of the material. Another is that most of the positive residual stresses are concentrated to the same depth. This occurs especially when using a medium hard core.
The martensite formed by conventional induction heating using a low-alloyed steel will almost immediately reach the full carbon content, i.e., 0.5%-0.6%.